From Discworld MUD Wiki
Skullcap is a small flowering plant in the mint family, often with small purple or blue flowers. Some species are reported to have a medicinal use.
Lush, dark green leaves with inconspicuous blue flowers.
or (Genua only)
Skullcap is a plant that reaches around knee-height with hairy, square stems and downy leaves. It puts forth many lavender coloured flowers.
It is poisonous to consume.
Skullcap can be found in the following locations: